The present invention is concerned with pipelaying methods and particularly with underwater pipelaying where the water is covered by an ice sheet.
It has previously been proposed to fabricate a pipe string, lay it on the top surface of the ice sheet, cut a slot in the ice sheet adjacent the pipe string, and to provide a series of gantry-like pipe lowering devices which span the trench and carry an underwater guideway down which the pipe is lowered through the slot in the ice to the sea bottom.
It is also known to lay underwater pipelines by a bottom pull method. In conventional bottom pull methods, pipe lengths are fabricated into a pipe string on shore and attached by a winching cable to a pulling station on a far shore or a pull barge located on the water surface; by which the string is pulled along the water bottom to the far shore. The pull cable in this instance may be supported on the water surface by floats to reduce the length of pull cable along the bottom of the channel.
The problem with the conventional bottom pull method in ice-covered water where the string length is great, is that ice cover prohibits the use of floats. The present invention seeks to provide a method by which long lengths of pipeline may be laid underwater, underneath an ice surface, by a bottom pull method.